Deepwater site shifts from gusher to underwater laboratory

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BP’s Gulf of Mexico gusher may finally be dead, but its months-long release of oil and gas has created quite an in situ oceanic laboratory that scientists will be studying for years. Even as scientists from the National Oceanic and Atmospheric Administration (NOAA) quibble with those from Woods Hole Oceanographic Institution in Massachusetts over the existence of oil plumes, a team of researchers has recently returned from the Deepwater site with what they hope is a treasure trove of information about how large quantities of hydrocarbons behave deep beneath the ocean surface.

A better understanding of the effects of large quantities of butane, ethane, methane and propane on aquatic life will "give us better insight into the anthropogenic aspects of [drilling], which is significant given that deepwater drilling will continue for some time," says John Kessler, an oceanographer at Texas A&M University College of Geosciences.

During an initial 10-day trip in June, the researchers focused on the methane component of Deepwater’s hydrocarbon soup of oil and gas. They wanted to know where the methane (which, by weight, made up 40 percent of the gusher) was going and how it was cycling. They studied a 12.5-kilometer radius around the well, later determining that "insignificant" quantities of methane were being released into the atmosphere and that the majority was being dissolved into the Gulf.

Kessler and Valentine last week published a paper in Science based on their first trip to the Deepwater site. The report highlighted how the respiration of natural gas (in particular, the combination of ethane, butane and propane) controlled about 70 percent of the oxygen reduction observed in the deep hydrocarbon plumes around the blown out well, Kessler says. "It’s not uncommon to see ethane and propane coming out of a leak along with methane, but this was a much higher abundance than you would see from natural seepage from the sea floor," Kessler says. It’s unclear at this point what was causing the other 30 percent, but most likely it was the respiration of oil compounds, he adds.

For the second excursion, Kessler, Valentine and their team served as methane measurement experts as part of NOAA’s larger operation to study and document the movement of underwater plumes of oil and gas. Kessler and Valentine wanted to see to what degree the plumes had decomposed (if at all) and measure the rate of this biodegradation. "There are several plumes, the majority of which are about two-thirds of the way down from the ocean’s surface, at about 1,000 meters," Kessler says. The researchers observed that the plumes were for the most part still at the same locations found during the first trip to the site. Now they will study the data they collected to determine how the compositions of these plumes compare with their compositions in June. In addition to returning to their original 12.5-kilometer radius, the researchers also found plumes that had propagated to the west of this area.

The researchers initially expressed some concern that the oxygen removed from the waters around the Deepwater site could become hypoxic (or oxygen depleted), creating "dead zones" that can be harmful to marine life. Kessler now says, "I would be very surprised if this led to another dead zone in the Gulf." Such zones are typically found closer to land, at depths of 20 to 100 meters and occur when about 60 percent of the oxygen is removed from a patch of water, he adds. In June, the researchers saw a 20 percent reduction in oxygen at the Deepwater site, a condition that should improve due to the ocean’s currents and the fact that the well is no longer leaking.